A method and device for distance measurement are described in PCT Application No. WO 97/09638. The device functions according to the FMCW principle (frequency-modulated continuous wave) and for this purpose, includes a signal source in the form of a tunable microwave oscillator which produces a linear frequency-modulated microwave signal. The frequency-modulated signal is transmitted via a transceiver and reflected against an object whose distance to the device is to be determined. The signal reflected back from the object is received by the transceiver and mixed in a demodulator with the signal intended for emission. Based on the propagation time of the received signal on its way from the transceiver to the object and back and the frequency change of the signal to be transmitted brought about in the meantime by the frequency modulation, a difference between the frequencies of the two signals is produced so that the demodulator outputs a demodulation signal with a frequency corresponding to this difference. Since the frequency modulation of the microwave signal takes place in a linear manner, the frequency of the demodulation signal is proportional to the distance between the device and the object, so that the distance can be determined directly from the frequency of the demodulation signal in an analyzing unit.
If, in addition to the actual target object, there are reflecting interference objects located in front of and behind it, a frequency spectrum is obtained for the demodulation signal which no longer allows the determination of the distance to the target object by pure frequency measurement. Plug-in connectors in the line path for the frequency-modulated signal, the edge of an antenna transmitting the signal or the bottom of a tank when a fill level is measured, for instance, act as interference objects. It is possible to subject the frequency spectrum to a Fourier analysis in the analyzing unit in order to be able to separate frequencies originating from various objects. However, due to the periodic linear, e.g., triangular or sawtooth-shaped, frequency modulation of the high-frequency signal, the Fourier transform must take place in a window of time as a result of which the frequency resolution is limited. Particularly when the distance to the target object is very small, the frequency of the demodulation signal is very low so that the ratio of the period length to the length of the window of time is less than one and may thus be too low for a Fourier transform in any case.